Manny is using a computer model to investigate the effects of substitution mutations. The model presents Manny with a long base
sequence for a gene. Manny then selects one base for a mutation, such as by substituting cytosine, "C", for adenine, "A". Manny completes many trials of mutations. In most trials, the result of the substitution mutation is either no effect, or a polypeptide in which one amino acid is altered. In one trial, however, the result is a truncated polypeptide. Many of its terminal amino acids are missing.
What is the most likely explanation for the truncated polypeptide.
A. The substituted base caused a frameshift mutation.
B. The mutation introduced a stop codon in the middle of the gene.
C. The mutation introduced a start codon in the middle of the gene.
D. The substituted base altered the genetic code.
The most likely explanation for the truncated polypeptide -due to the substitution of cytosine for adenine- is that mutation introduced a stop codon in the middle of the gene.
Explanation:
A codon consists of three nucleotides -in the RNA chain- whose order determines a specific amino acid. Not all codons code for amino acids, as there are termination codons, also called stop codons, which are UAG, UGA and UAA.
If in Manny's computer model, the substitution of cytosine by adenine produced a termination codon, the synthesis of a peptide is stopped prematurely resulting in a truncated peptide.
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